One particular form of such oxidation is the combustion of a fuel with air, particularly with an excess of air to effect complete combustion.
In order to reduce the formation of oxides of nitrogen (NOx) when a fuel, e.g. gaseous hydrocarbons such as natural gas and/or hydrogen, is combusted with air, it is desirable to employ fuel/air mixtures of such composition that the adiabatic flame temperature is relatively low, desirably below about 1300.degree. C. For many applications this means using a composition that is so rich in air that normal combustion is unstable and may not be self-sustaining. Catalytic combustion wherein a mixture of the fuel and air is passed through a bed of a combustion catalyst, enables such problems to be overcome.
One application wherein catalytic combustion is desirable is in gas turbines. At initial start-up of a gas turbine, a mixture of the fuel and air, preheated, for example by a pilot burner, to a temperature typically of the order of 600.degree.-800.degree. C. when the fuel is methane or natural gas, is fed, normally at superatmospheric pressure, e.g. at a pressure in the range 2 to 20 bar abs., to the inlet of the combustion catalyst bed. Combustion is effected at the catalyst surface forming a gas stream at elevated temperature. There is a rapid rise in the temperature of the catalyst bed to about the adiabatic flame temperature, typically about 1200.degree. C., when the catalyst lights-off. The point at which this occurs is associated with the pre-heat temperature and the catalyst activity. Until light-off occurs, the solid temperature rises exponentially along the bed length. The average temperature of the gas mixture increases more gradually as the gas mixture passes through the bed reflecting the increasing degree of combustion of the mixture. When the temperature of the gas mixture reaches a value, typically about 900.degree. C., at which homogeneous combustion commences, there is a rapid increase in the gas temperature to about the adiabatic flame temperature. When operating a gas turbine with catalytic combustion, when combustion has been established, it is usually desirable to decrease the preheating of the feed, e.g. to the temperature, typically about 300.degree.-400.degree. C., corresponding to the discharge temperature of the compressor compressing the air and fuel.
It is seen therefore that the catalyst has to exhibit catalytic activity at a relatively low feed temperature but has to withstand heating to relatively high temperatures of the order of 1000.degree. C. or more without loss of that low temperature activity.
Also, in gas turbine operation using catalytic combustion, the catalyst not only has to be able to withstand high temperatures, but also withstand the thermal shock of rapid temperature changes resulting from repeated stopping and starting of combustion. Also gas turbines are usually operated using high gas flow rates. These conditions impose severe restraints on the materials that can be utilised as the catalyst.
Combustion catalysts used under less severe conditions have commonly employed one Group VIII metals and/or oxides thereof supported on a suitable refractory support material. Examples of such metals and oxides that have been proposed include platinum group metals, such as platinum, palladium, or rhodium, or mixtures thereof, or iron, or nickel, in the metal or oxide form. We have found that for applications involving adiabatic flame temperatures above about 1000.degree. C., those catalysts are unsuitable. Thus in order to obtain a satisfactory activity the catalytically active material has to exhibit a high surface area; at the temperatures that are liable to be encountered, the aforementioned catalysts rapidly lose activity as a result of thermal sintering giving a decrease in the surface area and/or as a result of the active material having an appreciable vapour pressure at such temperatures with consequential loss of active material through volatilisation, particularly where the gas stream has a high velocity gas stream.